# Very high brightness and power LCLS-II hard X-ray pulses

## TL;DR

This paper demonstrates the feasibility of generating extremely bright, high-power, and narrow bandwidth X-ray pulses in the 4-8 keV range using the LCLS-II facility, enabling advanced scientific experiments.

## Contribution

It introduces a novel method to produce high-brightness, high-power X-ray pulses with narrow bandwidth using two electron bunches and monochromator delay compensation.

## Key findings

- Achieved 0.65 TW peak power X-ray pulses with 15 fs duration.
- Generated third harmonic pulses with 8-12 GW peak power.
- Over 94% of X-ray power within near transform limited bandwidth.

## Abstract

We show the feasibility of generating X-ray pulses in the 4 to 8 keV fundamental photon energy range with 0.65 TW peak power, 15 fs pulse duration, $9\times10^{-5}$ bandwidth, using the LCLS-II copper linac and hard X-ray (HXR) undulator. Third harmonic pulses with 8-12 GW peak power and narrow bandwidth are also generated. High power and small bandwidth X-rays are obtained using two electron bunches separated by about 1 ns, one to generate a high power seed signal, the other to amplify it by tapering the magnetic field of the HXR undulator. The bunch delay is compensated by delaying the seed pulse with a four crystals monochromator. The high power seed leads to higher output power and better spectral properties, with $>$94\% of the X-ray power being within the near transform limited bandwidth. We discuss some of the experiments made possible by X-ray pulses with these characteristics, like single particle imaging and high field physics.